1. Field of the Invention
This invention relates to an end fitting for a circuit-interrupting device, such as a fuse, and more specifically, to an improved end fitting for a high-voltage fuse.
2. Brief Description of the Prior Art
As a class, high-voltage fuses generally comprise an insulative housing containing a fusible element. The fusible element is attached at one end to a metallic ferrule attached to the housing, and at the other end, to a movable arcing rod or arcing contact which is held in a normal first position and restrained from moving by the fusible element. The arcing rod is in turn electrically connected by sliding contacts, cables, or the like, to a ferrule on the other end of the housing. Springs or the like are often contained within the housing for biasing the arcing rod for movement away from the fusible element. The fuse is connected into a high-voltage circuit by way of appropriate fuse mountings which mechanically and electrically connect to the ferrules. Should an over-current condition in the circuit occur, the fusible element fuses or melts, thus permitting the spring to move the arcing rod away from its normal position. Such movement stretches or elongates the resulting high-voltage arc, which occurs where the element melts, and causes that arc to interact with an arc-extinguishing medium through which the arcing rod moves. This interaction causes the evolution of deionizing, cooling and swirling gases by the arc-extinguishing material to ultimately extinguish the arc. The arcing rod then comes to rest at a second remote location.
Two specific types of the above generally-described high-voltage fuse are so-called non-dropout fuses and dropout fuses. In a non-dropout fuse, the above action occurs following an over-current condition but the fuse remains stationary in the fuse mounting. Various schemes for determining whether such a non-dropout fuse has operated have been devised. See, for example, commonly-filed, co-pending, commonly-assigned patent application, Ser. No. 740,930, filed Nov. 11, 1976, and other applications cited therein.
A dropout fuse has its internal parts so arranged that, upon operation thereof, the arcing rod or a part carried thereby (often termed a striker pin) exits at least partially from an end of the fuse. Such fuses may carry, at one end thereof, a thin metallic seal which is pierced by the striker pin and through which the striker pin partially exits after fuse operation. The protruding striker pin operates a mechanism on the fuse mounting adjacent the point of striker pin exit, to release the fuse from such mounting and to permit the fuse to rotate downwardly (i.e., drop out), thus giving a visual indication that the fuse has operated.
Clearly, a fuse designed for dropout operation could be utilized in a non-dropout mounting, wherein the protruding striker pin would not effect fuse dropout, but could give a visual indication of fuse operation without such dropout action. However, the striker pin is generally slender and not readily visible from a distance or under low light conditions. Accordingly, the use of a fuse basically intended for dropout operation in a non-dropout mounting has proven less than satisfactory insofar as a visual indication of fuse operation is given.
Non-dropout fuses, or fuses intended for dropout operation used in a non-dropout manner, may have blown fuse indicators. Such indicators have been included as a part of the fuse. Accordingly, if the fuse is discarded the blown fuse indicator is similarly discarded. This, of course, adds to the cost of the fuse.
Another problem noted with prior art fuses primarily intended for dropout operation involves the tendency of the striker pin, upon fuse operation at extremely high currents, to be driven completely from the fuse. Moreover, inasmuch as the striker pin is made of conductive metal, it may initiate short circuit or arcing conditions with respect to adjacent energized live parts which it contacts. The consequences of this problem may be especially acute where dropout fuses are used in non-dropout mountings in an enclosure.
When fuses primarily intended for dropout operation are used in enclosures, another problem may arise. Specifically, following piercing of the seal by the striker pin, gases, generated by the arc, may be vented in great quantities into the enclosure. These gases may include metallic vapors. The enclosure, of course, contains energized and grounded parts in proximity, and such gases may ultimately decrease the dielectric strength of the air between proximate live parts within the enclosure, resulting in a flashover or other damaging arcing conditions within the enclosure.
The state of the art is also such that, for a given voltage and current, manufacturers usually make fuses of two different constructions, one for indoor, non-dropout use and the other for outdoor, dropout use. This necessitates the provision of two constructionally different, but functionally similar, fuses depending upon whether dropout (outdoor) or non-dropout (indoor) use is desired, and results in increased manufacturing costs.